The invention relates to a hollow high-voltage insulator, which has an insulating body, with a hollow support element made of a thermosetting composition, and a potential control device. The invention also relates to a process for producing a hollow insulator of this type.
A hollow insulator of the foregoing type is used to allow current or voltage on high-voltage-carrying parts to be reliably measured by means of measuring transducers. A hollow insulator of this type is also used for example to allow high voltages to be conducted into a transformer. In the first case, the measuring transducer is arranged in the hollow space of the hollow insulator, one side of the measuring transducer being connected to the high-voltage-carrying part and the other side of the measuring transducer being connected to a measuring instrument or to ground. In the second case, a current conductor is for example led from a high-voltage-carrying line via the hollow space of the hollow insulator into the transformer.
The support element of the hollow insulator may be provided on its outer side with a cladding comprising shields. Silicone rubber has proven to be a successful material for the shields. The cladding of silicone rubber is thereby case solidly bonded to the thermosetting composition of the support element. This is also referred to as a composite insulator.
The thermosetting composition of the support element is decisive for the mechanical stability of the hollow insulator. A thermosetting composition is understood as meaning a highly polymeric material which is closely crosslinked up to the decomposition temperature and at lower temperatures is energy-elastic, and even at high temperatures does not have viscous flow. The glass transition temperature of a thermosetting composition always lies above 50xc2x0 C.
Examples of thermosetting compositions are phenolics, aminoplastics, epoxy resins acrylic, and alkyd resins, as well as unsaturated polyester resins.
When using a hollow conductor for measuring or leading through high voltages or currents, there are inevitably very short distances between the parts being insulated, which are at very different potentials. Regions with critical field strengths are formed, at which flashovers or discharges can easily take place and can lead to the destruction of the hollow insulator or the device on which the hollow insulator is arranged. To avoid such phenomena, it is known from Hxc3x9cTTE, Taschenbxc3xccher der Technik [technology pocket books], Springer Verlag Berlin, Electrische Energietechnik [electrical power engineering), volume 2: Gerxc3xa4te [devices], 29th edition 1978, section 2.1.3.6, to design leading-through current conductors or lead-throughs in general as what are known as capacitor bushings with potential control. In that case, an insulating body made of hard paper, soft paper or casting resin, which contains concentrically arranged cylindrical conductive coverings, is applied directly to the current conductor to be led through. The conductive coverings become shorter from the inside outward and control the potential distribution between the conductor and ground.
Reference is had, in this context, to European published patent applications EP 0 029 164 A1 and EP 0 032 690 A2, which disclose high-voltage lead-throughs of this type with capacitive potential control inserts.
It has also been known for controlling the potential of lead-throughs in the interior of a hollow insulator to provide control electrodes which are electrically bonded to the fittings by which the hollow insulator is fastened. The potential distribution between the led-through conductor and ground can also be controlled in this way.
If capacitor bushings with control inserts are used, the control electrodes must be disadvantageously applied directly to the conductor in a complex and expensive process. Such a process is not required when a current conductor is led through a hollow insulator. However, for controlling the potential, the control electrodes must then be subsequently arranged in the interior of the hollow insulator, involving additional installation effort. This disadvantageously increases the production costs for a hollow insulator. Moreover, both configurations for potential controllers, or generally for potential control device, disadvantageously require additional installation space.
German patent No. DE 32 08 358 C2 also discloses a casting resin insulator in which capacitive field control inserts are cast into the casting resin body of the insulator as potential control device. For this purpose, first of all a preform with successively step-shaped transitional regions is cast. After removal from the casting mold, its circumferential surface is provided with an electrically conductive covering and subsequently, in a second casting operation, is encapsulated with an outer casting resin sheath. Since it is necessary to work with two casting molds and, moreover, many separate working steps are required, the process described is complex and cost-intensive, with the result that the casting resin insulator obtained in this way is disadvantageously very expensive.
The object of the present invention is to provide a hollow insulator and a production method which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and wherein the hollow insulator can be produced in a particularly simple and low-cost process and the corresponding fabrication method is appropriately configured.
With the above and other objects in view there is provided, in accordance with the invention, a hollow high-voltage insulator, comprising:
an insulating body with a hollow support element made of a thermosetting composition, and a potential control device encapsulated with the thermosetting composition of the support element;
the potential control device being at least partially encoiled with fibers, and the support element built up by alternating insertion of the potential control device, coiling on of the fibers, and simultaneous or subsequent application of the thermosetting composition.
In other words, the potential control device is encapsulated with the thermosetting composition of the support element and at least partially encoiled with fibers.
The invention is in this respect based on the fact that the support element of a composite insulator is produced by curing a blank of the still soft thermosetting compositions. This is because it was recognized that, in this way, the potential control device can be arranged in the hollow insulator by being processed simultaneously with the soft thermosetting composition to form the blank. The joint processing takes place in this case by building up the blank layer by layer by alternating insertion of the potential control device, coiling with fibers and simultaneous or subsequent application of the thermosetting composition. It is also referred to as the filament-winding process. After the curing of the thermosetting composition, which, as known, takes place by a heat treatment, the potential control device is cast, i.e. solidly bonded, with the thermosetting composition of the support element. The support element is at the same time reinforced with fibers.
Neither the complex application of the potential control device to the conductor to be led through nor an additional installation effort for the potential control device to be subsequently introduced into the interior of the hollow insulator is required according to this novel invention. The invention combines the installation of the potential control device and the production of the support element into a single operation. Furthermore, no additional space in the interior of the hollow insulator is taken up by the potential control device encapsulated with the thermosetting composition of the support element.
The use of a thermosetting composition reinforced with glass fibers has been found to be particularly advantageous for the mechanical stability of the support element. Other insulating fibers, such as polyester or aramid fibers, can also be used. The latter are to be used for high strengths of the support element.
A particularly suitable thermosetting composition is epoxy resin.
For the electrical bonding of the potential control device, it is of advantage if the potential control device is encapsulated with the thermosetting composition in such a way that part of the potential control device is still freely accessible, i.e. is not covered by the thermosetting composition. Such a freely accessible location allows the remainder of the potential control device, lying inside the thermosetting composition, to be easily electrically bonded. If the potential control device is arranged entirely inside the thermosetting composition, the electrical bonding of the potential control device must be performed via a conductor led out from the thermosetting composition.
In an advantageous embodiment of the invention, the potential control device comprises a layer of electrically conductive material. In this way, a capacitive potential control can be achieved. It goes without saying that semiconducting material can also be used.
With a rotationally symmetrical design of the support element, for example as a circular cylinder or in a conically tapering form, it is also of advantage if the layer of the conductive material is formed into a tube, which may also be conically designed, with the center point in the longitudinal axis of the rotationally symmetrical support element. In this way, an effective potential dissipation control is achieved for a centrally led-through current conductor.
In a further advantageous embodiment of the invention, the potential control device comprises a plurality of tubes each made of the layer of conductive material, arranged in the rotationally symmetrical support element concentrically about the longitudinal axis of the support element and offset with respect to one another in a step-like manner. Such an arrangement allows both fine potential control and capacitive voltage measurement. In the latter case, the capacitance of the potential control device is led to the voltage measurement in an insulated manner.
It is favorable for production if the conductive layer is a metal foil, for example made of copper or aluminum. Metal foils of this type are commercially available inexpensively and can easily be processed with the thermosetting composition.
In order that no excessive potentials occur at the ends of the layers of metal foil in the hollow insulator, the end of the metal foil is advantageously rolled in or flanged. This avoids a sharp-edged transition between the metal foil and the matrix of the thermosetting composition.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing a high-voltage hollow insulator having an insulating body, with a hollow support element made of a thermosetting composition, and a potential control device, the method which comprises:
at least partially encoiling the potential control device in a filament-winding process, whereby a blank of the support element is formed by alternating insertion of the potential control device, coiling on of fibers, and simultaneous or subsequent application of the thermosetting composition;
encapsulating the potential control device with the thermosetting composition by heat treating the blank; and
curing the thermosetting composition and thereby forming the support element.
In other words, a blank of the support element is formed from the potential control device and the still soft thermosetting composition, the potential control device is encapsulated with the thermosetting composition by heating the blank, and the thermosetting composition is cured, thereby forming the support element.
The blank of the support element is produced by what is known as the filament-winding process, in that fibers are coiled onto a shaped body with simultaneous or subsequent application of the thermosetting composition, with the potential control device being at least partially encoiled. The simultaneous application of the thermosetting composition takes place for example by using glass fibers impregnated with the thermosetting composition.
For introducing the potential control device, the layer may in this case be advantageously applied to the required regions as the first part-layer on the shaped body. This layer may comprise a metal foil or some other conductive material.
In this way, it is easily possible for a plurality of conducting or semiconducting layers to be incorporated in such a manner that they are arranged one behind the other, in order to obtain with the potential control device a finer dissipation control of the potential.
The invention additionally offers the advantage that no mechanical or installation-related requirements have to be taken into account in the structural design of the potential control device. The structural design of the potential control device is for the most part only dependent on electrical influences.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a hollow insulator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.